The Ulster Unionists said today they would not be "bullied" into backing a deal to complete the Northern Ireland peace process, amid intense pressure on the party from Washington and London to support a vote to devolve police and justice powers to Stormont.

But, ahead of a vote on the policing deal today, the UUP leader, Sir Reg Empey, rejected the overtures. "We are prepared to go forward and look to the future but not under the cosh of all this blackmail and bullying," he said.

The UUP party executive confirmed last night that it would vote against the devolution of policing and criminal justice powers to Belfast. The unanimous decision means that the once-mighty UUP, which governed Northern Ireland from 1921 until direct rule was imposed in 1972, will be the only member of the four-party power sharing executive that will vote no today.

The Democratic Unionists and Sinn Féin, which brokered the agreement on policing last month, will join the SDLP in voting for the deal.

The widow of the first Police Service of Northern Ireland officer murdered by dissident republicans today appealed for an end to political bickering at Stormont.

Kate Carroll, whose husband Stephen was shot dead by the Continuity IRA last March, said it was time the politicians put aside their divisions in the interest of peace.

In a call to a local radio station, Carroll said: "This morning has been very, very hard for me, and I would just ask everyone in Stormont to please get on with their job."

In a direct message to the Ulster Unionist party, she added: "I am pleading on this day that is so important to me that it's not worth it. Life is too short. It is heartbreaking that I have to get on this morning to please ask the politicians to get on with their job."

The decision by the UUP to oppose what London describes as the final piece in the jigsaw of the 1998 Good Friday agreement will come as a blow to Cameron. The Tory leader supports the devolution of policing powers to Belfast but appears to have failed to persuade his political allies in Northern Ireland to follow his lead.

The prospect of a UUP no vote has also caused alarm in the US. Hillary Clinton, the US secretary of state, yesterday pleaded with Empey in a phone call to support the deal. The Guardian understands that the White House was so concerned that the US economic envoy to Northern Ireland, Declan Kelly, also persuaded Bush to intervene.

The former president telephoned Cameron last Friday to ask him to plead with Empey to endorse the deal. While the UUP does not have enough votes to scupper the deal, political leaders in the US fear a no vote from the UUP could undermine support for the settlement within the DUP and among the wider unionist community.

"There was a feeling that a conservative-to-conservative conversation was the right way to go about this," said one source familiar with the transatlantic negotiations. "This conversation was borne out of the concern that Empey is holding out."

Another source familiar with the contact said: "This is the most active thing George W Bush has done in his post-presidency period. He has been incredibly restrained and diplomatic since leaving the White House. He has maintained radio silence."

One source familiar with thinking on Northern Ireland on both sides of the Atlantic added: "The fact that George W Bush has decided to intervene is really significant. He was interested in the peace process as president and appointed an envoy. It is a general sign of how concerned people are in the US about what David Cameron is up to."

Owen Paterson, the shadow Northern Ireland secretary, confirmed on BBC Radio 4's Today programme this morning that Bush had phoned Cameron. He would not give any details of the call but said the former president had had "a very constructive and friendly conversation" with the Tory leader.

A Tory source said: "It was a positive conversation. David underlined his commitment to the agreement and said we are doing all we can to support it. But he said that we cannot force Sir Reg to vote for it. George Bush thanked David and said: 'I can see you are engaged.'"

Cameron has faced pressure in recent weeks over his decision to form an electoral pact with the UUP, which could provide him with crucial support in Westminster if the general election leads to a hung parliament. The pact has prompted fears in Washington that the Tory leader might abandon the even-handed approach to Northern Ireland that was adopted by John Major.